The removal of particulate matter from air streams is typically done now with baghouses or electrostatic precipitators and from liquids with screens or filters. Baghouses usually employ fabrics which serve as the support for the buildup of a filter cake. Periodically, the filter cake is shaken or blown loose from the fabric and collected, and the cycle is repeated.
Typical pressure drops are on the order of 10-20 cm WC (water column). As might be expected, there is some loss of efficiency after the cake is removed during the cleaning cycle and there tends to be a relatively high pressure drop during the latter phase of filtration just prior to cleaning. Furthermore, baghouse fabrics may blind due to lodging of fine particles or sticky materials in the pores of the fabric.
In addition to commercial baghouses and electrostatic precipitators. research and development work has been done on recirculating, granular-bed filters. These filters comprise a bed of granules in which the granules are recycled out the bottom of the bed, cleaned, and returned to the top. Dirty gas flows from side to side or from bottom to top, counter-current to the granule movement.
The removal of solid particles from the gas stream is vital to the successful operation of gas turbines in which coal or other ash forming fuel is burnt. An example is found in pressurized fluidized bed combustion (PFBC) of coal, in which the combustion products, at pressures typically of 5 to 20 atmospheres, exhaust through a gas turbine.
The advantages of Pressurized Fluidized-Bed Combustion (PFBC) in a combined cycle mode to produce electricity include potential overall efficiencies greater than 40 percent, and control of SO.sub.x, and 40 x emissions well below EPA's New Source
Performance Standards (NSPS).
As part of their PFBC program, a major developmental thrust of the U.S. Department of Energy (DOE), has been high temperature, high pressure (HTHP) particulate removal to meet turbine requirements and Environmental Protection Agency (EPA) standards of particulate emissions. Customarily, at least two separate stages of filtration are used: (1) a cyclone prefilter to scavenge large particulates (at least 20 micrometers in diameter); and (2) a primary filter to remove fine dust. The filtration requirements are set by protection of the gas turbine from erosive damage and by EPA/NSPS emission constraints.
Fabric filters arrayed in a baghouse comprise one of the technologies of choice in contemporary developments. The baghouse can be an expensive component (around 25 percent of the capital cost of an FBC installation) and also can pose some significant operational expenses in maintenance. While much progress has been made, the goal of achieving high collection efficiency at high temperature and pressure is yet to be reached with such filters.
The present novel concept of a rotating drum filter provides very high efficiency at relatively low pressure drop. The concept is based upon filtration through fibrous media that support the buildup of chain-like dust agglomerates (dendrites) which has been shown to be one of the most effective means for collection of micrometer and submicrometer particulates from gas streams. Fibrous media filters differ from fabric filters (baghouses) because the dust is deposited mainly on sites within the interior of the fiber bed, while in a fabric filter the dust forms an external cake on the surface. Fibrous media filtration provides the advantage of high efficiency at high gas flow rates. On a comparable efficiency basis, pressure drop for a fibrous filter is lower than for a baghouse.
The chief reason for the lack of wide acceptance of fibrous media filtration in the industrial market is related to the need for frequent cleaning or regeneration of the fiber bed. Consequently, fibrous filters have been employed only for nonregenerable applications such as residential furnace filters, respirator masks, and emergency filters for radioactive particle leaks. The present invention provides convenient continuous regeneration, and maximizes the advantages of dendrite filtration.
It has been found that the dendritic capture may be increased and the pressure drop decreased by increasing voidage. This leads to lower operating costs. One of the advantages of a fiber filter over a granular filter is the ability to develop and control this high voidage. The term voidage is intended to mean the percentage of a particular space that is empty of solids. It is calculated by determining the volume of the filter occupied by fibers, V.sub.f, as by dividing the mass of fibers by the density of the fiber material. The volume of empty space is then the total filter volume less the volume of fibers, V.sub.T -V.sub.f, and the voidage is the empty volume expressed as a fraction of the total volume, ##EQU1##
The high voidage is or loose packing phenomenon. It has been found that high-aspect-ratio fibers tend to nest in a rather rigid, high voidage array when they are loosely poured into a container. The nesting is a matter of degree. For capturing fine particles in the 1-20 micrometer range, fibers in the range of about 0.075 to 2 mm diameter and aspect ratios of above about 20 are preferred in the present invention. The voidage appears to vary linearly with aspect ratio of the fibers.
The nesting of fibers also provides a second advantage over the packing of granules in the granular-bed filters. The fiber nests tend to be quite rigid compared with the loose granules. Thus, dendritic formations contributing to good capture are retained in the cohesive fiber bed during operation. On the contrary, dust captured between bed granules can be lost by the downward movement of the loose granules in a granular bed filter.
The fibers may be made of any useful material including both organic and inorganic materials. They may capture particulate material by purely physical means or they may react chemically with a particulate material or gas. The fibers may also be catalytic or be coated with inert, reactant, or catalytic material. For example, metal fibers may have a catalytic coating to convert SO.sub.2 in flue gas to SO.sub.3, or a lime coating that can react with the SO.sub.2 to produce a CaSO.sub.4 deposit on the fiber.
Refractory fibers, as well as metal fibers, can be used for high temperature applications. For example, catalytic cracking of high boiling hydrocarbons to gasoline fractions can take place at about 500 C with fibers made or coated with modified, hydrated alumina silicates. Deposited carbon can be removed by burning in air in the regenerator.
An advantage of the present invention is the high face velocity that is possible for effective filtration at low pressure drop. Face velocities of 200 fpm are possible compared to velocities of 2-4 fpm for bag filters.
The present invention provides reliable operation at high temperatures, such as 2000.degree. F. and above, and at high pressures, such as 350 psig and above, as well as at lower temperatures and pressures. Other advantages include operation with lower pressure drops than are usually obtained with known apparatus, higher reliability, and the ability to operate continuously without interruption for cleaning.
High temperature, high pressure (HTHP) gases from operations such as pressurized fluid-bed combustion (PFBC), integrated gasification combined cycle (IGCC), and direct coal-fired turbines require very efficient particle control. The present invention provides such control.